The present invention relates to a semiconductor nonvolatile memory for use in a computer, an office automation machine or the like.
1. Field of the Invention
The present invention provides a semiconductor nonvolatile memory for use in a computer or the like wherein an erasing electrode and a writing electrode are provided adjacent to a floating gate electrode through respective tunnel insulator films, thereby improving the reliability of the semiconductor non-volatile memory.
2. Description of the Prior Art
Semiconductor non-volatile memories are widely applied to computers and the like since they are smaller in size than other types of memory and enable a high-speed reading operation. FIG. 2 is a sectional view of a typical electrically erasable and programmable non-volatile memory (hereinafter referred to as an "EEPROM"). On the surface of a P-type semiconductor substrate 11, N.sup.+ -type source and drain regions 12 and 13 are formed, and a floating gate electrode 15 is provided thereon through a gate insulator film 14. In addition, a control gate electrode 16 for controlling the potential of the floating gate electrode 15 is provided through an insulator film 102. The transfer of electric charge to and from the floating gate electrode 15 is effected through a tunnel oxide film 17 provided on the drain region 13. For example, in the case where the thickness of the tunnel oxide film 17 is about 200 .ANG., when a voltage which is 20V higher than the potential of the drain region 13 is applied to the control gate electrode 16, a strong electric field is applied to the tunnel oxide film 17, causing electrons to flow from the drain region 13 into the floating gate electrode 15. To extract electrons out of the floating gate electrode 15 into the drain region 13 reversely to the above, it suffices to apply a voltage to the drain region 13 which is 20V higher than the potential of the control gate electrode 16. Since the channel conductance between the source and drain regions under the floating gate electrode 15 which serves as a gate electrode changes in accordance with the amount of electric charge accumulated or stored in the floating gate electrode 15, it is possible to read out data.
For example, this kind of prior art is disclosed in U.S. Pat. No. 4,203,158, entitled "ELECTRICALLY PROGRAMMABLE AND ERASABLE MOS FLOATING GATE MEMORY DEVICE EMPLOYING TUNNELING AND METHOD OF FABRICATING SAME" by Dov Frohman-Bentchkowsky, etal.
In the case of an EEPROM in which a strong electric field is applied to a tunnel oxide film to rewrite data as shown in FIG. 2, the application of a strong electric field leads to wear and deterioration of the tunnel oxide film, so that repetition of the rewriting operation results in breakdown of the EEPROM.